Display device comprising a light guide with electrode voltages dependent on previously applied electrode voltages

ABSTRACT

A display device has row ( 5 ) and column ( 6 ) electrodes and a movable element ( 3 ) and means ( 17 ) for supplying voltages to the electrodes. The means supply, in operation, such voltages to the electrodes that use is made of the memory effect of the movable element. More in particular, the electrodes are, in operation, supplied with “on”, “off” and “hold” voltages. Simultaneous application of “on” voltages turns a pixel on, simultaneous application of “off” voltages turns a pixel off. Application of a “hold” voltage on either of the electrodes preserves the state of the pixel.

BACKGROUND OF THE INVENTION

The invention relates to a display device comprising a light guide, amovable element and selection means to locally bring said movableelement into contact with the light guide, said selection meanscomprising row and column electrodes and means for applying addressingvoltages to the row and column electrodes.

A display device of the type mentioned in the opening paragraph is knownfrom U.S. Pat. No. 4,113,360.

In said patent, a description is given of a display device comprising afirst plate of a fluorescent material, in which, in operation, light isgenerated and trapped (so that this plate forms a light guide), a secondplate which is situated at some distance from the first plate and,between said two plates, a movable element in the form of a membrane. Byapplying voltages to addressable electrodes on the first and secondplates and to electrodes on the movable element, the movable element canbe locally brought into contact with the first plate, or the contact canbe interrupted. A transparent contact liquid is present on the contactsurfaces. At locations where the movable element is in contact with thefirst plate, light is decoupled from said first plate. This enables animage to be represented. If the movable element is not in contact withthe light guide, it is in contact with the second plate.

For the proper functioning of the display device, it is important that,on the one hand, the contact between the light guide and the movableelement can be brought about and interrupted in an accurate and reliablemanner, but that, on the other hand, the design is simple and does notrequire much energy to operate.

It is an object of the invention to provide a display device of the typementioned in the opening paragraph, which provides a simple and yetreliable device.

To achieve this, the display device in accordance with the invention ischaracterized in that the selection means comprise means for applyingvoltages to the electrodes in dependence on a previously applied voltageor voltages on the electrodes.

In the known device, the position of the movable element, i.e. whetheror not it makes contact with the light guide is dependent on the appliedvoltages, and on said voltages only. The inventors have realized thatthe fact whether or not the movable element moves is dependent on theforces acting on the element. The forces acting on a movable element arenot only dependent on the applied voltages, but also on other forcesacting on the element and on its position vis-á-vis the electrodes. Saidposition is also dependent on the history of the element, i.e.previously applied voltages and position. The electric forces acting onthe movable element are non-linearly dependent on the distances betweenthe movable element and the electrodes. Because of the non-linearrelationship between force and distance, the device exhibits a memoryeffect. When the movable element is near one of the electrodes, only arelatively large voltage difference between the electrodes can move theelement to the other electrode. This, however, also means that once amovable element is in a certain position, it will stay in such aposition, even if the voltages applied are changed, provided that theydo not change to such a large degree that the movable element is movedto the other electrode. Since the device exhibits a ‘memory effect’,i.e. it is not only the momentary voltages applied which determinewhether or not the movable element moves, but this is also determined bypreviously applied voltages. Using this insight, one or a number ofadvantages can be obtained. The device can be simplified, and/or theaddressing voltages applied to the device can be simplified and/or theenergy required can be lowered and/or the reliability of the device canbe increased. Also grey levels can be made, as will be explained.

A preferred embodiment of the device in accordance with the invention ischaracterized in that the means for applying addressing voltages apply,in operation, a first set of voltages having a lower and an upper valueto a row electrode, and a second set of voltages having a lower and anupper value to a column electrode crossing the row electrode at acrossing area, the device being arranged in such a way that onlysimultaneous application of a lower value to the row electrode and anupper value to the column electrode, or vice-versa, changes the positionof the movable element at the crossing area.

Alternatively, a preferred embodiment of the device in accordance withthe invention is characterized in that the means for applying voltagesapply, in operation, a first set of voltages having a lower and an uppervalue to a column electrode, and a second set of voltages having a lowerand an upper value to a row electrode crossing the column electrode at acrossing area, the device being arranged in such a way that onlysimultaneous application of a lower value to the column electrode and anupper value to the row electrode, or vice-versa, changes the position ofthe movable element at the crossing area.

In these embodiments, application of an upper or lower value on oneelectrode (row or column) alone does not actuate the movable element atthe crossing area of the relevant row and column electrodes. Onlysimultaneous application of a lower value on one of the electrodes, andan upper value on the other, or vice-versa, will actuate the element atthe crossing area. Actuating the movable elements becomes very reliableby this measure. Small deviations of applied voltages do notinadvertently switch an element. Basically, only simultaneousapplication of two ‘on’ signals on row and column electrode(s) will turna pixel ‘on’ when it is ‘off’, and simultaneous application of two ‘off’signals on row and column electrode(s) will turn a pixel ‘off’ when itis ‘on’, as will be further explained in the description.

Preferably, the means for applying voltages apply, in operation, aturn-on addressing voltage to a row electrode, while simultaneouslyapplying addressing voltages to a number of column electrodes crossingsaid first electrodes to bring the movable element in contact with thelight guide at selected crossing areas of the row electrode, andsubsequently apply said turn-on addressing voltage to a second rowelectrode while, simultaneously applying addressing voltages to a numberof column electrodes crossing said first and second row electrodes tobring the movable element in contact with the light guide at selectedcrossing areas of the second electrode, the voltage at the first rowelectrode being in between the turn-on addressing voltage and a turn-offaddressing voltage, such that the position of the movable elements atthe crossing areas of the first row electrode does not change.

Alternatively, the means for applying voltages apply, in operation, aturn-on addressing voltage to a first column electrode, whilesimultaneously applying addressing voltages to a number of rowelectrodes crossing said first column electrode to bring the movableelement into contact with the light guide, at selected crossing areas ofthe first column electrode and subsequently apply said turn-on voltageto a second column electrode, while simultaneously applying voltages toa number of row electrodes crossing said first and second columnelectrode to bring the movable element into contact with the light guideat selected crossing areas of the second column electrode, the voltageat the first column electrode being in between the turn-on addressingvoltage and a turn-off addressing voltage, such that the position of themovable elements at the crossing areas of the first row electrode doesnot change.

A turn-on addressing voltage is understood to mean a voltage valuewhich, when combined with a given (turn-on) voltage at a crossingelectrode, results in bringing the movable element into contact with thelight guide at the crossing area.

Likewise, a turn-off addressing voltage is understood to mean a voltagevalue which, when combined with a given turn-off voltage at a crossingelectrode, results in releasing the movable element from the light guideat the crossing area.

These embodiments are based on the following recognition. When the firstrow or column electrode is supplied with an ‘on’ signal (turn-onvoltage), and a set of crossing electrodes is supplied with ‘on’ and‘off’ signals (‘off’ meaning ‘not on’), only those pixels correspondingto areas where electrodes cross and both carry ‘on’ signals will beturned ‘on’. A first line of picture elements is thus formed.

This step is thereafter repeated for the second (row or column)electrode to form a line of picture elements. However, the voltage atthe first row or column electrode is brought to a value between the ‘on’and an ‘off’ value. This means that the first line of picture elementsremains visible, i.e. ‘on’ and the information in said line ispreserved. In its simplest form, two lines of picture elements areformed in this manner. It will be clear that this scheme can be expandedto more than 2 lines.

The great advantage is that, while the second (or third etc.) line ofpicture elements is formed, the first (second etc.) line of pictureelements remains ‘on’. The total intensity of the light is therebyincreased substantially in comparison with arrangements in which (as,for instance, in classical CRTs) only one line of picture elements (orpixels) is activated (‘on’) at any one time.

This allows multi-line operation, i.e. more than one line (multi-line)is simultaneously active. The lines of picture elements (the videoinformation) could be written in columns or rows. This also allows greylevels to be made.

Preferably, the means for selection supply, in operation, such asequence of voltages that the percentage of time during which a givenrow or column electrode is active is approximately (within roughly 50%)uniform for all row or column electrodes, but does show a variation overthe device, depending on the distance between the row or columnelectrode and a nearest light input for the light guide, while thepercentage of time during which a given row or column electrodes isactive increases as the distance to a nearest light input increases.Light guides show absorption of light. This will cause a reduceduniformity of the light emitted by the display. By increasing thepercentage of time during which a row or column electrode is active,this effect can be counteracted to increase the uniformity in the imagedisplayed by the device.

A row or column electrode is active between the time when a turn-onvoltage has been supplied to the row or column electrode until aturn-off voltage has been supplied to said row or column electrode.

BRIEF DESCRIPTION OF THE DRAWING

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a cross-sectional view of a display device in accordance withthe invention;

FIG. 2 shows a detail of the display device shown in FIG. 1.

FIGS. 3A, and 3B show further details of the embodiments of the displaydevice shown in FIG. 1.

FIG. 4 is a plan view of the display device shown in FIG. 1.

FIG. 5A and 5B show embodiments of the display device in accordance withthe invention.

FIGS. 6A and 6B illustrate schematically the memory effect in a deviceaccording to an embodiment of the invention and how it is used.

FIGS. 7A and 7B illustrate schematically the memory effect in a deviceaccording to another embodiment of the invention.

FIG. 8 shows graphically the position of a movable element of FIG. 7 tofurther illustrate the memory effect.

FIG. 9 shows schematically the matrix structure used to form an image.

FIG. 10 illustrates schematically a possible addressing scheme togenerate grey levels.

The Figures are schematic and not drawn to scale, and, in general, likereference numerals refer to like parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a display device 1 in accordance with theinvention. Said display device comprises a light guide 2, a movableelement 3 and a second plate 4. Electrode systems 5 and 6 are arranged,respectively, on the facing surfaces of the light guide 2 and of thesecond plate 4 facing the movable element 3. In this example, the lightguide is formed by a light-guiding plate. The electrodes 5 and 6 formtwo sets of electrodes which cross each other at an angle of preferably90°. By locally generating a potential difference between the electrodes5, 6 and the movable element 3, by applying, in operation, electricvoltages to the electrodes and the movable element, forces are locallyexerted on the movable element, which pull the movable element againstthe light guide or against plate 4. The display device further comprisesa light source 9 and a reflector 10. Light guide 2 has a light input 11in which light generated by the lamp is coupled into the light guide 2.The lamp may emit white light, or light of any color, depending on thedevice. It is also possible that more than two lights are present, forinstance, a lamp on two sides or on each side of the device. It is alsopossible to use lamps of different colors sequentially to form a whitelight display. The light travels inside the light guide and, due tointernal reflection, cannot escape from it, unless the situation asshown in FIG. 2 occurs. FIG. 2 shows the movable element 3 lying againstthe light guide 2. In this state, a part of the light enters the movableelement. This movable element scatters the light, so that it leaves thedisplay device. The light can exit at both sides or at one side. In FIG.2, this is indicated by means of arrows. In embodiments, the displaydevice comprises color-determining elements 20. These elements may be,for example, color filter elements allowing light of a specific color(red, green, blue, etc.) to pass. In a preferred embodiment, a UV lampis used and UV light is fed into the light guide and leaves the lightguide and is incident on phosphor elements. The phosphor elementsexcited by the UV light emit colored light. The use of UV light andphosphor elements increases the efficiency of the display device.

FIG. 3A shows a further detail of the display device shown in FIG. 1.The movable element 3 is positioned between the light guide 2 and thesecond plate 4 by means of sets of spacers 12 and 13. Electrodes 5 and 6are covered by insulating layers 10 and 11 in order to preclude directelectric contact between the movable element 3 and the electrodes. Byapplying voltages to the electrodes and the movable element, an electricforce F is generated which presses the movable element against theelectrode 5 on the light guide 2. The electrode 5 is transparent. Thecontact between the movable element and the light guide causes light toleave the light guide and enter the movable element at the location ofthe contact. In the movable element, the light is scattered and part ofit leaves the display device via the transparent electrode 5 and thelight guide 2 and a part leaves through plate 4. If non-transparentelectrodes are used, as shown in FIG. 3B, these electrodes should beprovided next to the location where the optical contact between thelight guide 2 and the movable element 3 is brought about. It is alsopossible to use one set of transparent electrodes, the other beingreflective, which increases the light output in one direction. In FIG.3B, an embodiment comprising two electrodes is shown. This has thedrawback that more electrodes must be manufactured, thus causing anincrease in costs and a greater risk of picture errors. The use of onlyone of the electrodes shown has the disadvantage that each of theseelectrodes must generate a larger force, requiring higher voltages, andthe force is exerted asymmetrically.

FIG. 4 is a plan view of an embodiment of the display device shown inFIG. 1.

The electrodes 5 and 6 form a matrix structure. From a control unit 17,which comprises selection means, selection signals (electric voltages)are supplied to the electrodes 5 and 6 via the connections 15 and 16.This set of selection signals determines the set of potentials V₅ and V₆on the electrodes 5 and 6, which are preferably covered by an insulationlayer. Furthermore, a voltage V₃ can be applied to the element 3. Byapplying suitable potential differences to the electrodes 5 and 6 andelement 3, the movable element can be actuated, in operation, from andto the electrodes 5 and 6 at the location of the selected crossings ofthe electrodes 5 and 6. Electrodes 5 form column electrodes, i.e.electrodes extending in the ‘short’ direction of the rectangulardisplay, while electrodes 6 form the row or line electrodes, i.e.electrodes extending in the ‘long’ direction of the rectangular display.

FIGS. 5A and 5B show in more detail a part of the device shown in FIG.3A.

The force which is locally exerted on the movable element by a potentialdifference between the electrodes and the movable element is governed bythe potential differences, the distances between the electrodes and themovable element and the size of the surface area of the electrodes. Themovable element can be actuated by these forces. The electrostatic forceF which occurs between two electrodes (or between an electrode and themovable element) is, in the absence of static charges, approximately:

F=½ε₀(V/(d+∈d _(i)/ε_(i)))² .S

where F is the force, V is the potential difference between the element3 and an electrode 5 or 6, d is the distance between the outer, facingsurfaces of the element 3 and the electrodes 5 or 6 and d_(i) is thethickness of any layer (e.g. layers 51, 52, 53, 54 in FIG. 5A) on theelectrodes and/or element 3, ε_(i) is the dielectric constant for such alayer and S is the surface area of the electrodes. In the absence ofother forces, switching voltages of the order of 10 to 100 V can be usedto switch the movable element, i.e. cause it to locally make contactwith the light guide or interrupt the contact with the light guide.

Actually, two electro-static forces are acting on each element, oneforce (force F₁) being dependent, amongst others, on the difference inpotential between element 3 and electrode 5 and the distance betweenelement 3 and electrode 5 (V₃−V₅), and one force (F₂) being dependent onthe difference in potential between element 3 and electrode 6 (V₃−V₆)and the distance between element 3 and electrode 6.

The total electrostatic force acting on element 3 in FIG. 5A is:

F _(total) =F ₁ −F ₂ =C((V ₃ −V ₅)²/(d ₅₂/ε₅₂ +d ₅₃/ε₅₃)²−(V ₃ −V₆)²/(d+d ₅₁/ε₅₁ +d ₅₄/ε₅₄))²)

where C is a constant.

Depending on the total magnitude and direction of the electrostaticforce, the element 3 will be actuated or not, i.e. move or not. Thetotal electrostatic force acting on movable element 3 will change sign(thus changing from a force directed towards the element to a repulsiveforce) when

(V ₃ −V ₅)²/(d ₅₂/ε₅₂ +d ₅₃/ε₅₃)²=(V ₃ −V ₆)²/(d+d ₅₁/ε₅₁ +d ₅₄/ε₅₄)²

In the absence of other forces(e.g. elastic forces) in the situationdepicted in FIG. 5 V₃−V₆ must be larger than V₃−V₅ (by a factor(d+d₅₁/ε₅₁+d₅₄/ε₅₄)²/(d₅₂/ε₅₂+d₅₃/ε₅₃)² to actuate the movable element.Likewise, when movable element 3 is in an upward position, i.e. close toelectrode 6, V₃−V₅ must be a factor(d+d₅₂/ε₅₂+d₅₃/ε₅₃)²(d₅₁/ε₅₁+d₅₄/ε₅₄)² larger than V₃−V₆ to move theelement. This means that the fact whether or not the movable element 3is actuated will not only be dependent on the voltages applied, but alsoon the position of the movable element vis-á-vis the electrodes, andsaid position is dependent on previously applied voltages, i.e. thehistory of the element. Thus a memory effect occurs. FIG. 5B shows anembodiment in which layers 51 and 52 are not present.

The invention is based on the recognition that this memory effect ispresent.

FIGS. 6A and 6B illustrate the memory effect. FIG. 6A shows that, att=0, movable element 3 is close to electrode 6, separated by a distanced₁ from said electrode 6. No light will leave the movable element, i.e.the pixel is ‘off’. The movable element is separated from electrode 5over a relatively large distance d₂+d₃ At t=t₁, a pulse is applied toelectrodes 5 and 6, reducing the difference in voltage between movableelement 3 and electrode 6 and increasing the voltage difference betweenmovable element 3 and electrode 5. This pulse is such thatd₁/ε₁×(V₃−V₆)>(d₂+d₃/ε₃)×(V₃−V₅).

This will cause the movable element to move into a position as shown atthe right hand side of FIG. 6. The movable element is in contact withplate 2 and thus light is extracted from the light guide and scattered,or in other words, the relevant pixel of the display is ‘on’. Att₁<t<t₂, the voltages at the electrodes can be maintained at 0, whilethe position of the element is maintained as it was after the pulse att₁. Although no voltages are applied to electrodes 5 and 6, the pixelthus remains ‘on’. A pulse at time t₂ reducing the difference involtages between the electrode 5 and movable element 3 and increasingthe voltage difference between electrode 6 and movable element 3 willbring the movable element back to its original position, turning thepixel ‘off’. FIG. 6B shows an important aspect of the memory effect. Inthis Figure, at t=t₁, a pulse is given on electrode 6 which reduces thevoltage difference between element 3 and electrode 6. No pulse is givenon electrode 5. However, the reduction in voltage difference V₃−V₆ isnot big enough to move the movable element. The relevant pixel willtherefore stay ‘off’. At t=t₃, a negative pulse is given to V₅. Again,however, the applied voltages will not cause the element to be moved, sothe relevant pixel will stay ‘off’. Hence, only the simultaneousapplication of an ‘on’ pulse on both electrodes 5 and 6 will switch theelement at the crossing to an ‘on’ position.

FIGS. 7A and 7B show schematically another embodiment of the invention.In this embodiment, movable element 71 is formed of or by a flexibleelement, which is either conducting and in electrical contact withelectrodes 6, or is provided with electrodes 6 on its surface 72 facingelectrodes 5. In this embodiment, ‘movable’ means that the element mayincrease in thickness. At t=0, the voltage difference V₆−V₅ (FIG. 7A) isapplied The voltage difference has such a magnitude that the attractiveelectrostatic force is balanced by a counter-elastic force. This elasticforce is dependent on the elongation Δx.

Thus an electrostatic force F₁ attracts surface 72 towards electrode 5,whereas a counteracting elastic force F₂ retracts the surface 72 in theopposite direction. Due to the fact that the force F₁ is non-linearlydependent on the elongation Δx, a memory effect can be obtained.

At t=0, the voltage difference V₆−V₅ equals such a value (V_(s)) thatthe movable element exhibits a small elongation, but is separated fromthe insulating layer 61. The pixel is thus ‘off’. At t=t₁, a large pulseV_(p) is applied between electrodes 5 and 6. The electrostatic force isthereby increased so that the movable element is elongated and surface72 touches layer 62 so that the pixel is ‘on’. Even though the voltagedifference is thereafter (t₁<t<t₂) reduced to the original value V_(s),the movable element stays in the ‘elongated’, i.e. ‘on’ status. When, att=t₂, a reduced voltage pulse V_(p)′ is applied, the elastic forcebecomes larger than the attractive electrostatic force and the movableelement is brought back to its original position. The movable elementthus shows a memory effect, because the position of the movable elementis not only dependent on the actual value of the voltage between theelectrodes, but also on previously applied voltages. FIG. 7B illustratesthe situation in which, at t=t₁, a pulse is given only to electrode 6 sothat the voltage is increased to V_(p)′, which value is larger thanV_(s) but smaller than V_(p). In between t=t₁ and t=t₂, two pulses canbe applied to electrode 5, namely a positive pulse which reduces thevoltage difference to V_(p)″ and a negative pulse which increases thevoltage difference to V_(p)′. The Figure shows on the left-hand sidethat V₆ in this embodiment has three different values, an upper value(V_(6m)), a middle value (V_(8m)) and a lower value (V_(6l)). Likewise,V₅ in this embodiment has three different values, an upper value(V_(5m)), a middle value (V_(5m)) and a lower value (V_(5l)).

FIG. 8 illustrates the memory effect by means of a graph in which theforces F₁ and F₂ are depicted as a function of the elongation Δx and asa function of the applied voltages (for force F₁). The Figure shows thatforce F₂ is linearly dependent on the elongation Δx, and theelectrostatic force is non-linearly dependent on the elongation Δx. Itis this non-linearly dependency which allows a memory effect. When F₁ islarger than F₂, the element will be elongated and surface 72 will movetowards the electrode 5, when F₁ is smaller than F₂, the surface 72 willmove away from electrodes 5. As FIG. 8 shows for V=V_(p), theelectrostatic force F₁ is larger than the elastic force for all valuesof Δx. This means that the electrostatic force will always win and theelongation will increase until the movable element is in contact withlayer 62, depicted by point B in FIG. 8, so that applying such a highvoltage difference always turns the pixel ‘on’. A small voltagedifference V_(p)′ means that the electrostatic force is always smallerthan the elastic force except for a very small elongation depicted by A′in FIG. 8. Applying such a voltage difference therefore always turns thepixel ‘off’. For voltage differences <V_(p), the situation is asfollows. If the pixel is in the ‘off’ position, it will stay ‘off’. Atmost, the elongation will slightly increase to the situation depicted byA′, or decrease slightly, but the elongation will not increase to pointB. If the pixel is ‘on’ (point B in FIG. 8), the pixel will stay ‘on’ aslong as V>V_(p)′ since in these conditions the electrostatic force isalways larger than the elastic force. Therefore, the position of themovable element is not only dependent on the applied voltages per se butalso on the position of the movable element and hence on previouslyapplied voltages. In a device according to the invention, the selectionmeans comprise means for applying voltages to the electrodes independence on a previously applied voltage or voltages on theelectrodes.

Table 1 indicates the values for the voltage difference as a function ofthe voltages applied to electrode 5 (V₅) and electrodes 6 (V₆) and theaction which will follow (pixel is turned on or off). This table holdsfor both embodiments shown in FIGS. 6A, 6B and 7A, 7B.

TABLE 1 Voltage difference V₆-V₅ as a function of voltages applied toelectrodes 5 and 6 voltages applied V₅ = V_(5h) V₅ = V_(5m) V₅ = V_(5l)to 5 and 6 ‘off-signal’ ‘hold-signal’ ‘on-signal’ V₆ = V_(6h) V_(s)V_(p)″ V_(p) ‘on-signal’ no action no action pixel turned ‘on’ V₆ =V_(6m) V_(p)′″ V_(s) V_(p)″ ‘hold-signal’ no action no action no actionV₆ = V_(6l) V_(p)′ V_(p)′″ V_(s) ‘off-sign’ pixel turned ‘off’ no actionno action

Table 1 makes it clear that no action occurs if either V₅ or V₆ isV_(5m) V_(6m), respectively, i.e. a ‘hold-signal’ is given to bothelectrodes. Simultaneous application of ‘on-signals’ will turn a pixelon, while simultaneous application of ‘off-signals’ will turn a pixeloff. As far as the actions are concerned, the table can be simplified totable 2 below (note that this also means that, instead of threedifferent voltages being applied to electrodes 5, only 2 differentvoltages V_(5h) and V_(5l) can be applied to the electrodes 5 with thesame results):

TABLE 2 Voltage difference V₆-V₅ as a function of voltages applied toelectrodes 5 and 6 voltages applied to 5 and 6 V₅ = V_(5h) V₅ = V_(5l)V₆ = V_(6h) V_(s) V_(p) no action pixel turned ‘on’ V₆ = V_(6m) V_(p)′″V_(p)″ no action no action V₆ = V_(6l) V_(p)′ V_(s) pixel turned ‘off’no action

Table 2 shows that, when V₆=V_(6m), the status of the pixel ispreserved, whatever the value for V₅. Pixels which are ‘off’ stay ‘off’and pixels which are ‘on’ stay ‘on’. For the rest of the description,V_(6m) will be described as V_(6hold), i.e. the value for which thestatus of each pixel is held, i.e. not changed, V_(6h) will be describedas V_(6on), i.e. the value for V₆ for which a pixel could be turned‘on’, provided the value for V₅ is V_(5l), and V_(6l) will be describedas V_(6off), i.e. the value for V₆ for which a pixel could be turned‘off’, provided the value for V₅ is V_(5h).

Since the scheme in table 2 is simpler than that shown in table 1, itillustrates a preferred embodiment of the invention, namely one in whichone of the sets of voltages has only two values, a lower and an uppervalue (although in practice, small deviations may of course occur in thesupplied voltages) and the other set comprises three values, a lower, amiddle and an upper value.

An important aspect of the memory effect as explained above is thatmulti-line addressing can be applied.

FIG. 9 illustrates schematically multi-line addressing.

At t=0, all values for V₆ are made equal to V_(6off) and all values V₅are made equal to V_(5off). At all crossings of the electrodes, i.e. atall picture elements, the movable element will not be in contact withthe light guide. Thus, no light is emitted. At t=t₁, the voltage on thetop row electrode, i.e. V₆, is changed to V_(6off). Video signals areapplied to the column electrodes V₅, V₅′, V₅″ etc. Some columnelectrodes are supplied with ‘on’ voltages V_(5l) (V_(5on)), whileothers are supplied with ‘off’ voltages V_(5h) (V_(5off)). At thecrossing areas of the column electrodes supplied with V_(5on) with thetop row electrode, the movable element will be brought into contact withthe light guide and light will be scattered. At the other crossing areasno light will be emitted. Subsequently, the second of the top rowelectrode (V₆′) is supplied with voltage V_(6on), while the voltage onthe top row electrode is changed to V_(6hold). The column electrodes aresupplied with video information corresponding to the second line of theimage. This second line of picture elements is formed, while the pixelsof the first line that were switched on are still emitting light. Next,the third row electrode is made ‘active’, i.e. supplied with V_(6on),while the first and second row electrodes are held at a voltageV_(6hold), i.e. remain active. In the further description, the processin which information is written on a line is referred to as ‘madeactive’, ‘activation’ or ‘switching’, when a line has been activatedand, until it is blanked, such a line is referred to as ‘active’. Whenthe third line of picture elements is formed (made active), the firsttwo lines are still emitting (active). In a simple scheme, this processis repeated until N lines are written, one of the N lines is blanked andan N+1 line is switched, whereafter another one of the N lines isblanked and an N+2 line is switched. Although, in this example, theimage is formed line by line and the lines are activated, going from topto bottom, it will be clear that any sequence of activation of the linesmay be used. For instance, sequences wherein subsequently the 1^(st),6^(th), 11^(th), 2^(nd), 7^(th), 12^(th) lines etc. are activated arepossible. This is done by supplying an ‘off’ voltage to the electrodecorresponding to said line and at the same time supplying ‘off’ signalsto all electrodes crossing said electrode.

Grey scales in the picture elements can be made by regulating thepercentage of time each crossing area is emitting light (duty cyclemodulation).

Although a number or even all lines may be active for some time, onlyone line may be switched (being made active or blanked) at any one time.

FIG. 10 illustrates this. The zigzag line at the upper half of theFigure illustrates the voltages being supplied to a first line. At t=0,a voltage V_(on) is supplied to a row electrode 6. This will activatethe line corresponding to said row electrode. Simultaneously, videoinformation (i.e. voltages V_(on) for those crossing areas where thepixel is to be turned on) are supplied to the column electrodes crossingsaid electrode. At t=t₁, the electrode is supplied with a voltageV_(off) and simultaneously all electrodes crossing the electrode aresupplied with a voltage V_(off). This will blank the line. This blankingtakes time τ_(s). After a short waiting time τ_(d), the line isactivated again. The video information can then be changed for eachelectrode crossing the relevant line electrode. Thus, the first time thepixel can be one, the second time 2τ off, the third time 4τ on, etc. Foran 8-bit grey scale, a complete cycle comprises, for instance, 8sub-periods of lengths 2, 4, 8, 16, 32, 64 and 128τ, two sub-periodsbeing separated by an “off-on” sequence taking τ_(s)+τ_(d)+τ_(s). Thetotal time each cycle takes is then (1+2+4+8+16+32+64+128) (=255)τ+8(2τ_(s)+τd). Since, at any one time, only one line may be switched(activated or deactivated), 8(2τ_(s)+τ_(d)) must be smaller than theline time.

The lower half of FIG. 10 indicates, by means of time slots for a firstelectrode 1, a second electrode 2 and a third electrode 3, two differentschemes of supplying voltages to the three electrodes. These schemes for3 active lines, indicated by arrows, show that there are some timeperiods between a (activation) and d (deactivation). At these timeperiods, no line is switched.

Absorption of light occurs in the light guide. By regulating time τ_(d)or the time periods indicated by the arrows, it is possible to regulatethe percentage of time that a line is active. In a preferred embodimentof the invention, the time τ_(d) and/or the number of time periodsindicated by the arrows are larger than at some distance of the lightinput. In this manner the percentage of time that light is emitted neara light input is less distant from the light input. Since, however, dueto absorption in the light guide, the intensity of the light is greatestnear the light input, a better uniformity is obtainable.

Summary, the invention may be described as follows.

A display device has row (5) and column (6) electrodes and a movableelement (3) and means (17) for supplying voltages to the electrodes. Themeans supply, in operation, such voltages (V_(5on), V_(5off), V_(6on),V_(6off), V_(6hold)) to the electrodes that use is made of the memoryeffect of the movable element. More in particular, the electrodes are,in operation, supplied with “on” (V_(5on), V_(6on)), “off” (V_(5off),V_(6off)) and “hold” voltages (V_(6hold)). Simultaneous application of“on” voltages turns a pixel on at the crossing area of the relevantelectrodes, while simultaneous application of “off” voltages turns apixel off. Application of a “hold” voltage on either of the electrodespreserves the state of the pixel. Multi-line addressing of the displaydevice is thereby possible. An important aspect is that grey levels canbe made.

It will be obvious many variations are possible that within the scope ofthe invention without departing from the scope of the appended claims.

What is claimed is:
 1. A display device comprising a light guide, amovable element and selection means to locally bring said movableelement into contact with the light guide, said selection meanscomprising row and column electrodes and means for applying voltages tothe row and column electrodes, characterized in that the selection meanscomprise means for applying voltages to the electrodes in dependence ona previously applied voltage or voltages on the electrodes.
 2. Thedisplay device as claimed in claim 1, characterized in that the meansfor applying voltages comprises: a first set of voltages having a lowerand an upper value to a row electrode, and a second set of voltageshaving a lower and an upper value to a column electrode crossing the rowelectrode at a crossing area, the display device being arranged in sucha way that only simultaneous application of a lower value to the rowelectrode and an upper value to the column electrode, or vice-versa,changes the position of the movable element at the crossing area.
 3. Thedisplay device as claimed in claim 1, characterized in that the meansfor applying voltages comprises: a first set of voltages having a lowerand an upper value to a column electrode, and a second set of voltageshaving a lower and an upper value to a row electrode crossing the columnelectrode at a crossing area, the display device being arranged in sucha way that only simultaneous application of a lower value to the columnelectrode and an upper value to the row electrode, or vice-versa,changes the position of the movable element at the crossing area.
 4. Thedisplay device as claimed in claim 1, characterized in that one of thesets of voltages is constituted by a lower and an upper value, the otherset of voltages being constituted by an upper, a middle and a lowervalue.
 5. The device as claimed in claim 1, characterized in that themeans for applying voltages comprises: a turn-on voltage to a first rowelectrode while simultaneously applying voltages to column electrodescrossing said first row electrode to bring the movable element intocontact with the light guide at selected crossing areas of the first rowelectrode, and subsequently the turn-on voltage to a second rowelectrode, while simultaneously apply voltages to said column electrodesto bring the movable element in contact with the light guide at selectedcrossing areas of the second row electrode, the voltage at the firstcolumn electrode having such a value that the movable elements at thecrossing areas of the first row electrode do not detach from the lightguide.
 6. The device as claimed in claim 1, characterized in that themeans for applying voltages comprises: a turn-on voltage to a firstcolumn electrode while simultaneously applying voltages to rowelectrodes crossing said first column electrode to bring the movableelement into contact with the light guide, at selected crossing areas ofthe first column electrode and subsequently apply the said turn-onvoltage to a second column electrode while simultaneously applyingvoltages to said row electrodes crossing said second column electrode tobring at selected crossing areas at the second column electrode themovable element in contact with the light guide, the voltage at thefirst column electrode having such a value that the movable elements atthe crossing areas of the first row electrode do not detach from thelight guide.
 7. The device as claimed in claim 1, characterized in thatthe selection means, in operation, such a sequence of voltages to therow or column electrodes that the percentage of time during which agiven row or column electrode is varied is such that the percentageincreases with an increasing distance between the relevant row andcolumn electrode and a light input into the light guide.